The present invention relates to a method and apparatus for measuring the waveform quality of signals that are transmitted from mobile stations of, for example, a mobile radio communication system using the CDMA scheme.
The standardization of a digital cellular system using the CDMA scheme is now proceeding in Subcommittee (TR45.5) of TIA/EIA under the leadership of Qualcomm, Inc. and standards for performance evaluations of transceivers are defined as IS-98 and IS-97. The present invention relates, in particular, to a method for measuring parameters of CDMA modulated mobile station send signals, such as a carrier frequency error, the carrier phase and the clock (symbol) phase (timing), and detecting therefrom the waveform quality defined in Standard IS-98 mentioned above.
There has already been proposed in U.S. patent application Ser. No. 08/825,502 entitled "Parameter Measuring Apparatus for Digital Quadrature Modulation Signals" (The corresponding Japanese patent application was not laid open to public inspection as of the priority date (Dec. 4, 1996) of this application.) a method for measuring, based on Standard IS-98, the waveform quality of mobile station send signals OQPSK (offset QPSK) modulated according to Standard IS-95. A description will be given first, with reference to FIG. 1, of the configuration of the apparatus.
The signal to be processed is down-converted to an IF signal, which is subjected to AD conversion by suitable sampling and stored as digital data in a memory. The IF digital signal is converted to a complex base band signal in a baseband converting part 11. The complex base band signal is input into a rough parameter estimating part 12, in which its parameter (the carrier frequency, for instance) is roughly estimated, then the thus estimated parameter is used to correct the complex base band signal in a correcting part 13, and the corrected base band signal is demodulated in a data detecting part 14. The demodulated data is provided via a time reference extracting part 15 to a reference signal generating part 16, wherein a reference signal is generated. The complex base band signal corrected in the correcting part 13 and the reference signal are used to estimate the parameter with high precision in a precise parameter estimating part 17. Finally, the complex base band signal corrected in the correcting part 13 is further corrected corresponding to the highly precise estimated value, and the resulting corrected signal and the reference signal are used to calculate the waveform quality in a waveform quality calculating part 19. The parameter estimation in the rough parameter estimating part 12 is made with such a degree of accuracy as to permit correct demodulation of the base band signal in the data detecting part 14.
The conventional method involves the data demodulation to extract the time reference. To perform this, it is necessary to estimate the parameter to such an extent as not to cause a demodulation error and correct the complex base band signal accordingly prior to the data demodulation. Hence, the occurrence of a demodulation error will cause an error in the extraction of the time reference, introducing an error in the subsequent parameter estimation. Furthermore, the prior art method employs plural parameter estimating parts and, upon each parameter estimation, entirely corrects the complex base band signal. If the signal is long, the calculating time will increase accordingly.